Handle cases when Div/Sub would be needed in shapes

[janimo]

chunk() and split() operations calculate the ouput dim by dividing the input dim.
slice() or narrow() for the last element(s) along a dim need to subtract from the dim lenght.

Div and Sub are not supported in the Nat based shape expression verifier so we need workarounds for these common operations.
For example a chunk with a known fixed number of chunks can be replaced by that many manual calls to slice()

[statusfailed]

Example of this: [https://github.com/hellas-ai/catgrad/blob/master/catgrad-llm/examples/gpt2.rs#L234]

[janimo]

chunk() can be handled when the chunk_size is known at build-time (which may be most of the time) see #252